The invention relates to an optical glass which, in particular, is suited for the preparation of transmissive optical components of a considerable size or of a considerable volume, respectively.
Prior art optical glasses offer a good homogeneity, good transmission characteristics in the desired spectral range and favorable Abbe numbers, however, the manufacture of optically transmissive components, such as lenses, prisms and windows of large sizes, e.g. having a diameter of 0.5 m or 1 m or a thickness of 50 mm, 100 mm or more, respectively, is very demanding under practical considerations.
For testing the polishing quality of highly precise astronomic mirrors, in particular of convex secondary mirrors, to this end usually in the interferometric testing devices often reference lenses, so-called matrices, are utilized. Common for this purpose are for example bi-convex lenses having at least one surface strongly shaped aspherically.
There are demanding requirements for the material of theses lenses with respect to the number of striae and to the homogeneity of refractive index. Preferably, a material having a low coefficient of expansion is utilized. In addition, herein a transmittance as high as possible is desired at the laser wavelength of 633 nm that is commonly utilized in testing.
Up to now, for such applications optical glasses, such as BK7 or FK5, are utilized. However, the manufacture of such large optical parts, such as of lenses having a diameter of 1 to 1.5 m or a thickness of 100 to 150 mm, respectively, is not possible therewith, at least not in an economically feasible way.
From German patent specification DE 1 596 860 and from German examined patent application DE 1 902 432, a glass ceramic is known that is marketed by the applicant under the trademark Zerodur®. However, such a glass ceramic is not suitable glass for the production of large optically transmissive optical components, since, due to the many dispersive centers generated by the crystalline inclusions, the transmission is impaired. Also the base glass has a slightly yellowish coloring which is negative for the transmission.